Highly dispersed cerium oxide subnano-particles containing coordinatively unsaturated (cus) Ce in a lower oxidation state (3 +) were deposited onto γ-Al2O3 using a pulsed arc-plasma process. The as-prepared and thermally aged Ce/Al2O3 catalysts were found to exhibit much higher activities for CO oxidation than those prepared using a conventional wet impregnation process. Characterization by HAADF-STEM, XAFS, XPS and FT-IR and kinetic analysis suggested that the surface abundance of cus Ce plays a key role in the unusual catalytic activity and deviation from the Mars–van Krevelen mechanism.